Rates of Reaction
7 marks · HardA researcher investigates the decomposition of hydrogen peroxide to oxygen gas. She runs two versions of the experiment: one at low pressure without a catalyst, the other at high pressure with a catalyst.
Describe what is meant by the term 'catalyst'.
Explain how increasing the pressure affects the rate of the decomposition of hydrogen peroxide when both hydrogen peroxide and oxygen are in the gas phase.
The table gives the results for two versions of the reaction:
| Experiment | Pressure | Catalyst | Time to collect 20 cm³ O₂ / s |
|---|---|---|---|
| 1 | low | none | 120 |
| 2 | high | manganese(IV) oxide | 20 |
Compare the time taken to collect the same volume of oxygen in the two experiments. Explain your answer in terms of the effects of both pressure and the catalyst.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) A catalyst increases the rate of a reaction and is chemically unchanged at the end. · must show Increases rate, Not used up/unchanged
(b) Increasing the pressure brings the particles closer together, making them collide more often, which increases the rate of decomposition of hydrogen peroxide in the gas phase. · must show Particles closer/more collisions, Increases rate of reaction
(c) In experiment 2 (high pressure and catalyst), the time to collect 20 cm³ O₂ is much less (20 s) than in experiment 1 (low pressure, no catalyst – 120 s). This is because increasing pressure increases collision frequency, making the reaction faster, and the catalyst lowers activation energy, making more collisions successful. Together they greatly increase the rate, so the gas is produced much more quickly. · must show Compares times (20 s vs 120 s, much faster in 2), Says both high pressure and catalyst speed up rate, Explains pressure: more collisions, Explains catalyst: lower activation energy/more successful collisions
Common mistakes: Not stating catalyst is unchanged; Forgetting to link faster collection to rate; Attributing more O₂ made to pressure/catalyst instead of just the rate
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Defines catalyst as a substance that speeds up reaction and is unchanged after.
M2 (b) · 1 mark · States increased pressure increases rate for gases/increases collision frequency.
M3 (b) · 1 mark · Explains pressure causes more frequent particle collisions.
M4 (c) · 1 mark · Compares times correctly (20 s vs 120 s, experiment 2 much faster).
M5 (c) · 1 mark · Explains effect of pressure: more collisions/more frequent collisions.
M6 (c) · 1 mark · Explains role of catalyst: more successful collisions/lowered activation energy.
M7 (c) · 1 mark · Links these effects to much faster rate/shorter time for O₂ collection.
Worked solution
a) A catalyst is a substance that speeds up the rate of a reaction but is not itself used up or changed permanently in the process.
b) Higher pressure in the gas phase brings the reacting particles closer together, increasing how often they collide. This leads to an increased rate of decomposition.
c) In experiment 1, it takes 120 seconds to collect 20 cm³ O₂, while in experiment 2, it only takes 20 seconds. Experiment 2 is much faster because the high pressure increases collision frequency between reactant particles, and the catalyst (manganese(IV) oxide) lowers activation energy, so more collisions result in reaction. The combined effects greatly increase the rate of oxygen production and shorten the time needed to collect the gas.